Wednesday, December 5, 2012

A Tiny, Glancing Blow

An oblique impact created a beautiful asymmetrical ejecta pattern on the farside highlands, photographed from LRO October 10. LROC Narrow Angle Camera (NAC) observation M1104509842L, spacecraft orbit 15079, resolution 1.2 meters per pixel over a field of view 696 meters wide. With the Sun high, sheer reflectance, hinting at rough and fresh terrain, is emphasized. In an earlier, closer observation below reflectance gives way to topography under a high angle of incidence [NASA/GSFC/Arizona State
University].

Sarah Braden

LROC News System

The ~220 meter diameter impact crater, located at 6.258°N, 215.101°E, just east of the unnamed crater highlighted in yesterday's Featured Image, was caused by an obliquely impacting asteroid or comet.

What is an oblique impact, and how do we know that this crater was formed by an oblique strike?

The term oblique impact implies an impact angle of 15 degrees or less. The impact angle is the angle between the surface and the vector that represents the direction of travel of the impactor.

A slightly closer look, under a greater angle of incidence (71.25°), at the rough ejecta and pressure wave pattern immediately beyond the north rim of the small glancing impact, from an earlier LROC NAC observation that unfortunately only overlaps the Featured Image at these points. The smaller grooves overlap those from the unnamed crater to the west (upper left). LROC NAC M118444752L, orbit 2589, January 18, 2010; resolution 1.09 meters from 52.43 kilometers [NASA/GSFC/Arizona State University].

The greater than 15 degree impact angle results in a number of diagnostic features including asymmetric ejecta and non-circular crater shapes. In the Featured Image, it is clear that this impact has asymmetric ejecta since the area immediately to the south of the crater is "missing" its high reflectance ejecta.

The asymmetry of the small impact, its missing south half, are clearly seen in this LROC Wide Angle Camera (WAC) mosaic of sequential LROC WAC observations in orbits 11104 and 11105, November 18, 2011; 62 meters resolution over a roughly 30 km-wide field of view [NASA/GSFC/Arizona State University].

The small crater's location, high amidst the highest elevations on the Moon, in the Farside Highlands Terrain (FHT), is indicated by the arrow at center, that also shows slope angles throughout the region are not as level as they might seem in close photography. LROC WAC Digital Terrain Model (DTM) hemispheric orthographic projection centered on 240° E (below) [NASA/GSFC/DLR/Arizona State University].

Due to the low angle of impact, the ejecta has more momentum in the direction of travel of the impact, which causes the asymmetric ejecta patterns. The area with the least ejecta is sometimes called the "zone of avoidance," and it indicates the impactor flight direction. In this case, the impactor was traveling from the south to the north when it hit the lunar surface (north is up in the Featured Image).